PURPOSE: To suppress the emission of hydrogen cyanide gas due to the combustion or thermal decomposition of a rigid plastic foam, by incorporating copper or a compound thereof in the rigid plastic foam.

CONSTITUTION: In preparing a plastic foam, e.g. a rigid polyurethane foam or urethane-modified polyisocyanurate foam, 0.05W5wt%, preferably 0.1W1wt%, based on the total weight of the foam, electrolytic copper powder as copper or cupric salt of an aliphatic carboxylic acid or cuprous carbonate as a copper compound is added to the raw material for the foam. The resultant mixture is then molded.